A deeper understanding of how clouds
will respond to a warming climate is one of the outstanding challenges
in climate science. Uncertainties in the response of clouds, and
particularly shallow clouds, have been identified as the dominant
source of the discrepancy in model estimates of equilibrium climate
sensitivity. As the community gains a deeper understanding of the many
processes involved, there is a growing appreciation of the critical
role played by fluctuations in water vapor and the coupling of water
vapor and atmospheric circulations. Reduction of uncertainties in
cloud-climate feedbacks and convection initiation as well as improved
understanding of processes governing these effects will result from
profiling of water vapor in the lower troposphere with improved
accuracy and vertical resolution compared to existing airborne and
spaceborne measurements. 1)

To respond to a wide range of airborne process studies, HALO can be rapidly reconfigured to provide either CH4 DIAL+HSRL, H2O DIAL+HSRL, or CH4 DIAL+H2O
DIAL measurements using three different laser transmitters and a single
multi-channel and multi-wavelength receiver. This paper will provide an
overview of the HALO program including advancements of new laser
technologies for airborne and spaceborne measurements of water vapor
and methane and preliminary CH4+aerosol measurements from recent airborne test flights. 3)

Science objectives:

• Use combined lidar profiles of water vapor, aerosols and clouds to better understand ...

• April 19, 2019: Whether
they’re idyllic floating cotton balls on an otherwise blue sky or
ominous grey swirls that block the sun, clouds all begin as an
invisible dot of water vapor. This elusive gas has been tricky to
measure and track – until now. Research scientists at
NASA’s Langley Research Center in Hampton, Virginia, have created
a new airborne instrument that can directly measure water vapor and
floating particles in the atmosphere. The new data will help check the
accuracy of satellite measurements, and improve weather and climate
forecasts. 4)

The instrument is called the High
Altitude Lidar Observatory (HALO). It uses light detection and ranging
(lidar), which works by shooting a pencil-thin laser beam through the
atmosphere. Light from the pulsed laser bounces off molecules and
particles suspended in the atmosphere, revealing what the human eye
cannot see. The intensity of the signal reflected back to the lidar
instrument gives the team the information they need to directly measure
water vapor, as well as aerosol and cloud profiles.

Figure 12: The HALO instrument
head, which houses the lidar instrument, is installed onto the DC-8
airborne science laboratory at NASA Armstrong Flight Research Center in
Edwards, California. The gold and blue casing holds the laser, optics,
detectors, and electronics, which are at the heart of the lidar (image
credit: NASA /Lauren Hughes)

- Water vapor is the most abundant
and potent greenhouse gas in the atmosphere. It warms the air by
trapping heat emitted from Earth, but also cools by forming bright
clouds that reflect heat radiated by the Sun. HALO’s data will
help scientists as they research the extent of each of these processes.

- HALO is a minifridge-sized modular
instrument that allows scientists to more easily measure either water
vapor or methane. This kind of technology is new at NASA, said Amin
Nehrir, a research scientist and HALO’s principal investigator.

- The ADM-Aeolus launched on Aug.
22, 2018, from Europe’s Spaceport in Kourou, French Guiana. It is
the first satellite to profile wind speeds on a global scale from
space, and it also collects aerosol data. HALO’s aerosol
measurements will be used to validate the satellite. Although
HALO’s water vapor measurements are not key to this particular
mission, they will give scientists a more comprehensive picture of the
atmosphere and help the team prepare for future airborne campaigns
dedicated to atmospheric dynamic processes.

Figure 13: The HALO system
electronics and diagnostic tools are integrated onto the DC-8 airborne
science laboratory at NASA Armstrong Flight Research Center in Edwards,
California. The lidar system control electronics are on the right hand
side of the rack. The large monitors on the left are used to display
real-time images of water vapor and aerosol profiles, which are used by
the science team to guide in-flight decisions and navigation. The
compact HALO instrument head can be seen directly behind the
electronics rack (image credit: NASA/Lauren Hughes)

Understanding water vapor

- HALO, and its ability to work with
other NASA systems that profile the atmosphere, “can provide a
huge leap in our understanding of atmospheric convective
processes,” Jeffrey Reid, a research meteorologist with the U.S.
Naval Research Laboratory Monterey, CA said. These processes are often
fueled by water vapor, which helps determine how clear, cloudy or
stormy the weather is.

- “From a weather and climate
perspective, understanding water vapor is the $50 problem,” David
Turner, a meteorologist with the National Oceanic and Atmospheric
Administration (NOAA) Earth System Research Laboratory in Boulder,
Colorado said. Understanding other gases in the atmosphere is more of
the $1 problem.

- Climate and
weather models aim to determine how many clouds cover a certain area of
land and what properties those clouds have, including size and
brightness, because that will affect Earth’s energy budget.
“Ultimately, climate is about the radiation balance at the top of
the atmosphere over time,” Turner said. And that radiation
balance is very sensitive to water in all of its phases, he continued.

- In the past, researchers would try
to use other types of observations, like radiosondes, which are
battery-powered devices that travel the atmosphere by weather balloon
to take stock of its surroundings, to infer what water vapor profiles
look like. “HALO makes a much more direct measurement of water
vapor,” Turner said.

- However, in order to use HALO
observations in weather prediction models, “We need a lot more
HALOs,” Turner said. HALO’s recent flight out of Armstrong
covers a very small section of the globe. “We could really use
that data in our models, but it’s not going to have as big an
impact as if you had 20 HALOs flying around the world,” Turner
said.

- HALO is 5 percent the volume and
20 percent the weight of LASE. Although NASA was able to measure water
vapor with LASE, “it was too big a beast to make it
economical,” Reid said.

- HALO, on the other hand, is a
smaller, modular instrument that can also be configured to measure
methane, which is also a potent greenhouse gas, in addition to
aerosols, and cloud and ocean profiles, Nehrir said. The modular set-up
allows the team to reconfigure the instrument in a matter of days.

- The HALO team demonstrated its new
methane and aerosol measurement capability in 2018 through a series of
test flights on the Langley Research Center-based B200 aircraft. HALO
flew over 100 flight hours as a part of the Long Island Sound Tropospheric Ozone Study (LISTOS), a multi-agency field mission designed to improve air quality forecasting tools in metropolitan areas.

- These test flights gave the team
confidence to continue working toward its ultimate goal of
demonstrating the new water vapor laser technologies on a similar
aircraft, and eventually in space.

Figure 14: HALO collected this
colorful data set showing aerosols, or tiny particles, suspended in the
atmosphere. The data were collected during the LISTOS field campaign
over Long Island Sound in the summer of 2018. As HALO sends its laser
beam down from an aircraft, it pains a two-dimensional map of what it
sees. Warm colors (reds and oranges) indicate a higher concentration of
aerosols and cool colors (greens and blues) show fewer aerosols. The
white specs toward the bottom of the image are caused by clouds, which
block the laser beam from reaching the surface and cause a black line
in lieu of data. The HALO team anticipates generating similar 2-D
images of the water vapor profiles below the aircraft during the
ongoing ADM Cal/Val mission (image credit: NASA /Amin Nehrir)

Looking to the future

- The effort to develop HALO has
spanned about three-and-a-half years. Within four-and-a-half years from
the start of the project, Nehrir and his team are planning to complete
five flight campaigns. “This has been an extremely aggressive
build and test schedule,” Nehrir said.

- Once HALO returns to Langley,
Nehrir and his team plan to reconfigure the instrument within one week
to measure methane and aerosols and integrate it on the NASA C-130
research aircraft out of NASA’s Wallops Flight Facility in
Wallops Island, Virginia. HALO will be the lidar instrument on the NASA
Earth Venture Suborbital-2 Atmospheric Carbon and Transport
(ACT)-America field mission, which is looking at quantifying carbon
emissions over the central and eastern parts of the U.S.

- HALO is also
preparing to fly on the NASA DC-8 jet in the summer of 2020 during an
international field campaign out of Cape Verde off the east coast of
Africa. This campaign brings together U.S. and European assets with
multiple aircraft to carry out a comprehensive validation of the
ADM-Aeolus satellite mission. Combining data sets from HALO and other
wind and cloud profiling instruments in this environment will help
scientists answer vital, outstanding questions on the biogeochemical
cycling of dust, Reid said. That cycling drives the genesis and
intensification of storms, which can eventually turn into major
hurricanes, Nehrir added.

- HALO is also looking to
participate in future severe storm and Arctic field campaigns, where
small variations in moisture can seed or prevent cloud formation
depending on the surrounding environment. These clouds have a large
impact on the Arctic radiation budget, which drives how quickly the
Arctic warms and releases methane from the melting tundra. HALO’s
ability to measure methane will help scientists understand where and
how much Arctic methane is entering the atmosphere.

The information compiled and edited in this article was provided byHerbert
J. Kramer from his documentation of: ”Observation of the Earth
and Its Environment: Survey of Missions and Sensors” (Springer
Verlag) as well as many other sources after the publication of the 4th
edition in 2002. - Comments and corrections to this article are always
welcome for further updates (herb.kramer@gmx.net).